Repeated loading of a wafer onto a stage can introduce offsets in the coordinates of a location on the wafer where measurements are taken. De-skewing is a method to correct the offset. De-skewing is often performed prior to taking metrology measurements of the wafer at different steps of wafer processing.
To de-skew a wafer, initial coordinates of two or more unique sites (“de-skew sites”) on a wafer are determined after the wafer is loaded onto a mechanical stage at a first time and subsequent coordinates of the two de-skew sites are determined after the wafer (or a different wafer) is loaded onto the stage at a subsequent time. A transformation matrix is calculated from the initial and the subsequent coordinates of the de-skew sites. The transformation matrix is used to convert between the initial and subsequent coordinates of a measurement location. Thus measurements can be taken at the same locations on the wafer with convenience and accuracy.
A conventional pattern recognition system can be used in the de-skew process. The conventional pattern recognition system uses a “recipe” that includes one image of each de-skew site of the wafer after one specific process (i.e., for a specific layer of the wafer). During certain wafer processes such as chemical mechanical planarization (CMP), pattern images of the de-skew sites may change so significantly that the pattern recognition system is unable to find the de-skew sites using one recipe. Thus conventionally the pattern recognition system uses a different recipe after each specific process (i.e., for each specific layer of the wafer).